This invention pertains to an improved process for the continuous production of aromatic carboxylic acids by the liquid-phase oxidation of alkyl aromatic hydrocarbons with molecular oxygen in the presence of an oxidation catalyst or catalyst system wherein an off-gas derived from the oxidation vessel comprising mainly water vapor and minor amounts of organic components is treated in a pollution control device and the aqueous effluent from the pollution control device is removed from the production system. More particularly, this invention pertains to such oxidation processes carried out in a columnar oxidation reactor provided with means to effectively remove water generated by the process with minimal solvent loss utilizing the energy of oxidation wherein the gaseous effluent stream from the water removal means is fed to a pollution control device and the aqueous effluent from the pollution control device is removed from the production system.
The liquid-phase oxidation of an alkyl aromatic hydrocarbon to an aromatic carboxylic acid is a highly exothermic reaction commonly carried out in a vented, intimately-mixed, columnar oxidation reactor. The oxidation process comprises continuously feeding, separately or in admixture, an alkyl aromatic hydrocarbon, fresh and/or recycled solvent or reaction medium, and catalyst components to the reactor to which a molecular oxygen-containing gas also is fed, normally at or near the bottom of the reactor. This process gas rises through the liquid contents of the reactor resulting in vigorous agitation of the reaction mixture and providing intimate contact between the alkyl aromatic hydrocarbon and the process solvent having dissolved therein the catalyst or catalyst components. The aromatic carboxylic acid produced is removed continuously through a lower exit port located at or near the base of the reactor as a solid in the solvent which also contains soluble catalyst components. After separation of the aromatic carboxylic acid product, the solvent is returned to the reactor.
Oxygen-depleted process gas along with a minor amount of solvent decomposition products, is removed through an upper exit port located at or near the top of the reactor. The heat of reaction also is removed through the upper exit port by vaporization of process solvent and water generated by the reaction. The oxygen-depleted process gas and the vaporized process solvent and water comprise the reactor off-gas which is typically condensed by means of one or more condensers to separate the solvent and water for recycling to the reactor. The condensed aqueous solvent may be subjected to a water removal step prior to recycling.
The described production system can be utilized in the manufacture of aromatic carboxylic acids at excellent production rates relative to the volume of the reactor. It is necessary that the production system include a means for the efficient removal of the excess water generated by the reaction since the water concentration must be held at an acceptable level, normally between about 3 and 20 weight percent, preferably between about 3 and 10 weight percent, for the reaction to continue at a reasonable rate. The reaction produces one mole of water per mole of carboxyl moiety produced. In addition, there are other by-product reactions that release water, i.e. the direct oxidation of the alkyl aromatic or direct oxidation of the solvent, and water may be added to the process for other reasons such as scrubbing off-gas for solvent recovery. Typically, water is removed by conventional distillation methods.
Direct distillation of the reactor off-gas to remove water has conventionally been employed utilizing the heat of reaction, as described in British Patent Specification 1,373,230 (Yokota et al.) and U.S. Pat. No. 4,914,230 (Abrams et al.), U.S. Pat. No. 5,723,656 (Abrams), and U.S. Pat. No. 5,463,113 (Yamamoto et al.) However, process limitations exist. Since the amount of distillate reflux determines the purity of the overhead distillate and the heat input to the distillation process determines the amount of reflux that the process can accommodate, the heat of reaction fixes both the amount of reflux and the purity of the overhead distillate. The heat of reaction alone is generally insufficient to obtain a desirable overhead purity that minimizes solvent loss. Therefore, direct distillation generally requires additional heat input. U.S. Pat. No. 5,510,521 (McGehee et al.) discloses an improved process wherein oxidation reactor off-gas is fed directly to the lower section of a removal column. A bottoms liquid of partially de-watered process solvent obtained from the lower section of the water removal column is returned to the upper section of the reactor, usually as a spray above the phase separation of the gas/liquid contents of the reactor. The spray of dewatered process solvent enriches the water content of the reactor off-gas to improve the efficiency of the water removal column without additional heat input beyond that of the heat of reaction.
A problem with the removal of water as liquid using direct distillation is that any such water also contains minor amounts of solvent organic by-products, requiring that the stream be treated as wastewater for removal of solvent and organic by-products due to the aforementioned oxidation reactions before release to the environment. The processes described by Yokota et al., McGehee et al., Abrams et al., Abrams, and Yamamoto et al. all remove water as liquid and thus require wastewater treatment for removal of solvent.
Thus, a need exists for a method to remove water from a carboxylic acid production process by a means which does not require wastewater treatment for removal of solvent and organic by-products before release to the environment. Providing such a water removal means would be especially advantageous if it improved process efficiency by recovering additional power using a power recovery device on oxygen-depleted process gas and water vapor streams.
The present invention provides for the removal and treatment of water of reaction as a vapor in a pollution control device, decreasing or eliminating the need for wastewater treatment of the stream for removal of solvent and organic by-products before release to the environment. These and other advantages are afforded by carrying out the oxidation of an alkyl aromatic hydrocarbon in a columnar reactor wherein the reactor off-gas is fed directly into a water removal column. A portion of the overhead aqueous vapors from the water removal column is removed from the top of the water removal column as a vapor distillate, with the remaining overhead aqueous vapors being refluxed to the fractionating zone of the water removal column. The combined vapor distillate and oxygen-depleted process gas are fed to a pollution control device for the destruction of solvent and organic by-products before exiting the process. Feeding a portion of the overhead aqueous vapor from the water column directly to the pollution control device reduces the required cooling utility duty and required heat transfer area to condense the water from all of the aqueous vapor stream removed from the water column.
Our invention thus provides a process for the continuous production of an aromatic carboxylic acid in a pressurized oxidation reactor by liquid-phase, exothermic oxidation of an alkyl aromatic hydrocarbon with an oxygen-containing gas in the presence of an oxidation catalyst and aqueous, C2-C6 aliphatic, monocarboxylic acid solvent which comprises the steps of:
(1) continuously feeding to a reactor alkyl aromatic hydrocarbon, aqueous, monocarboxylic acid solvent having oxidation catalyst dissolved therein, and an oxygen containing gas;
(2) continuously removing from the lower portion of the reactor product-containing liquid comprising aromatic polycarboxylic acid and the aqueous, monocarboxylic acid solvent having the oxidation catalyst dissolved therein;
(3) continuously removing from the upper portion of the reactor and feeding directly into the lower portion of a water removal column reactor off-gas comprising oxygen-depleted process gas and vaporized aqueous, mono-carboxylic acid solvent;
(4) continuously removing from the lower portion of the water removal column a bottoms liquid containing partially de-watered monocarboxylic acid solvent and returning to the reactor at least a portion of the bottoms liquid to the upper section of the reactor;
(5) continuously removing from the water removal column overhead a vapor stream comprised of oxygen-depleted process offgas, water and minor amounts of monocarboxylic acid solvent and organic by-products produced in the oxidation reactor;
(6) feeding a portion of the vapor stream of step (5) to a condenser to obtain (a) a vapor comprising oxygen-depleted process gas and (b) a liquid;
(7) feeding the liquid of step (6)(b) to the fractionating zone of the water removal column; and
(8) feeding (i) the remaining portion of the vapor stream of step (5) and (ii) the vapor of step (6)(a), to a pollution control device wherein the monocarboxylic acid and organic by-products present in vapor streams (i) and (ii) are destroyed to obtain an oxygen-depleted process offgas and aqueous vapor stream free or substantially free of organic compounds.
The oxygen-depleted process offgas and aqueous vapor stream free or substantially free of organic compounds obtained from step (8) may be, and normally is, removed from the aromatic carboxylic acid production system. Removal of the water of reaction from the process as a vapor in accordance with step (8) of our novel process avoids the necessity for wastewater treatment of the water of reaction for removal of solvent and organic by-products and reduces the required cooling utility duty and required heat transfer area to condense the water from all of the aqueous vapor stream removed from the water column.
A second and preferred embodiment of the present invention includes the recovery of power from high pressure vapor streams of step (5) and step (6)(a) by feeding those streams to a power recovery device. Thus, this second embodiment provides a process for the continuous production of an aromatic carboxylic acid in a pressurized oxidation reactor by liquid-phase, exothermic oxidation of an alkyl aromatic hydrocarbon with an oxygen-containing gas in the presence of an oxidation catalyst and aqueous, C2-C6 aliphatic, monocarboxylic acid solvent which comprises steps (1) through (7) described above in combination with the steps of:
(8.1) feeding (i) the remaining portion of the vapor stream of step (5) and (ii) the vapor of step (6)(a), to a power recovery device wherein the pressure of vapor streams (i) and (ii) is reduced and power is recovered resulting in power recovery and an effluent stream of reduced pressure; and
(8.2) feeding the effluent stream of step (8.1) to a pollution control device wherein the monocarboxylic acid and organic by-products present in the effluent stream of step (8.1) are destroyed to obtain an oxygen-depleted process offgas and aqueous vapor stream free, or substantially free, of organic compounds.
The second embodiment of our invention provides the additional advantage of increased power recovery in the process since the water generated in the pollution control devices, e.g., an oxidation reactor, which manifests itself in the form of additional aqueous vapor, is fed to a power recovery device.
A third and further preferred embodiment of the present invention includes preheating the high pressure vapor streams of step (5) and step (6)(a) prior to feeding those streams to a power recovery device. This third embodiment provides a process for the continuous production of an aromatic carboxylic acid in a pressurized oxidation reactor by liquid-phase, exothermic oxidation of an alkyl aromatic hydrocarbon with an oxygen-containing gas in the presence of an oxidation catalyst and aqueous, C2-C6 aliphatic, monocarboxylic acid solvent which comprises steps (1) through (7) described above in combination with the steps of:
(8.3) feeding (i) the remaining portion of the vapor stream of step (5) and (ii) the vapor of step (6)(a), to a preheater to increase the temperature of vapor feeds (i) and (ii) by at least 200xc2x0 C. to obtain a preheater vapor effluent:
(8.4) feeding the preheater vapor effluent from step (8.3) to a power recovery device wherein the pressure of the preheater vapor effluent is reduced and power is recovered resulting in power recovery and an effluent stream of reduced pressure; and
(8.5) feeding the effluent stream of step (8.4) to a pollution control device wherein the monocarboxylic acid and organic by-products present in the effluent stream of step (8.4) are destroyed to obtain an oxygen-depleted process offgas and aqueous vapor stream free, or substantially free, of organic compounds.
The third embodiment of the present process preferably utilizes a catalytic oxidation reactor as the pollution control device and the heat produced by the exothermic, oxidative decomposition of organic material in the oxidation reactor is utilized to provide the heat for the preheater. This preferred operation of the third embodiment utilizes the steps of:
(8.3) feeding (i) the remaining portion of the vapor stream of step (5) and (ii) the vapor of step (6)(a), to a preheater to increase the temperature of vapor feeds (i) and (ii) by at least 200xc2x0 C. to obtain a preheater vapor effluent:
(8.4) feeding the preheater vapor effluent from step (8.3) to a power recovery device wherein the pressure of the preheater vapor effluent is reduced and power is recovered resulting in power recovery and an effluent stream of reduced pressure;
(8.6) feeding the effluent stream of step (8.4) to an oxidation reactor wherein the monocarboxylic acid and organic by-products present in the effluent stream of step (8.4) are removed by exothermic, oxidative decomposition to produce a heated, oxygen-depleted process offgas and aqueous vapor stream free, or substantially free, of organic compounds; and
(8.7) feeding the heated, oxygen-depleted process offgas and aqueous vapor stream of step (8.6) to the preheater of step (8.3) to provide the heat required for the operation of the preheater.
The preferred operation of the third embodiment provides the following additional advantages: (1) increased power recovery due to the recovery of heat produced by the exothermic decomposition of organic materials in the catalytic oxidation reactor used as the pollution control device and (2) improved operability due to superheating of the feed to the power recovery device which avoids or reduces the likelihood of vapor condensation in the power recovery device and the mechanical and corrosion problems which may result from such condensation.